An electric vehicle has been considered as one of the most effective means of transportation because of the lack of pollution compared with vehicles employing internal combustion engines. However, electric vehicles suffer from various disadvantages, such as a long charge time for the battery and a limited range. Electric hybrid vehicles have been used in which an internal combustion engine is used along with a battery powered electric motor to combine the best features of the two types of propulsion. Such an electric hybrid consumes fuel while the engine is running and consumes electric power when the electric motor is running.
There are two types of hybrid vehicles: series hybrid and parallel hybrid. In a series hybrid vehicle, driving power is delivered by an electric motor that converts electrical energy stored in batteries into mechanical energy at the wheels. The battery is charged by a generator coupled to a conventional internal combustion engine. The combustion engine in a series hybrid vehicle is used to charge the battery when its state of charge falls below a predetermined threshold.
In a parallel hybrid vehicle, on the other hand, tractive propulsion is obtained simultaneously from the internal combustion engine and the electric motor. By an appropriate control strategy, the electric motor may be used in a generating mode to charge the battery when it is nearly depleted. A parallel hybrid arrangement provides a more efficient and direct transfer of mechanical energy from the engine and electric motor to the driving wheels than a series hybrid arrangement in which the mechanical energy is first converted to electrical energy by the generator and then converted to mechanical energy at the wheels.
There are four broad categories of parallel hybrid configurations: (a) a combination of traction forces, in which traction torques from the engine and electric motor are applied separately to different pairs of wheels and combined through their mutual interaction with the road; (b) combination of torques, where the driving torques from the engine and electric motor are mechanically combined (by a gear arrangement or belt) with a two-shaft arrangement before being applied to the transmission; (c) combination of torques wherein the engine and electric motor share a common propulsion shaft; and, (d) combination of speeds, in which the engine and electric motor operate at different speeds and input their respective driving torques into a complex gear system, which mechanically combines the torques before transmitting it to the differential. The present invention is a combination of speeds type.